In a process, such as a semiconductor process, in which thin-film growth or fine machining is performed, the temperature of a substrate greatly affects the quality of thin film and machining accuracy. Therefore, controlling the temperature of the substrate is very important, and accurate measurement of the temperature is required. Conventionally, a thermocouple, a fluorescent thermometer, or the like has been used for measurement of the temperature of a substrate. However, since the thermocouple and the fluorescent thermometer are brought into contact with the back surface of the substrate for measurement, a measurement error may arise if the state of contact is not good. Also, in the case of a plasma machining process in which thermal inflow occurs on the front surface side of a substrate, the temperature of the substrate itself cannot be measured.
In view of the above, a method for measuring the temperature of a measurement object in a non-contact condition has been demanded, and a method which utilizes interference of light has been proposed, such as that disclosed in Patent Document 1. In the disclosed method, the temperature of a measurement object is measured as follows. First, low-coherence light is split by a splitter into reference light and measurement light. The measurement light is caused to be incident on the measurement object and is reflected thereby, and the reference light is reflected by a mirror. The mirror is moved by a drive unit so as to change the optical path length of the reference light. The measurement light reflected by the measurement object and the reference light reflected by the mirror are caused to interfere, and the produced interference waveform is measured. Then, the temperature of the measurement object is measured from a change in the position of an interference peak caused by a change in refraction index with temperature or thermal expansion. Also, since an interference peak attributable to reflection on the front surface of the measurement object and an interference peak attributable to reflection on the back surface of the measurement object are obtained, the thickness of the measurement object can be measured. Examples of the light source include an SLD (super-luminescent diode), an LED, and a supercontinuum light source. Particularly, since supercontinuum light (SC light) has a flat spectrum of a wide band and has a short coherence length, conceivably, measurement accuracy can be improved by increasing resolution.